1. Field of the Invention
This invention relates to a metallic clad for coinage. More particularly, a planchet is formed by cladding a gold colored copper alloy to both sides of a copper alloy core that has a work hardening response and a dynamic softening response similar to that of the cladding layers.
2. Description of the Prior Art
A gold colored coin is highly desirable for distinguishing the coin from other denomination coins. The cost of the metal used for the coin should be low relative to the face value of the coin to enhance seigniorage. There are presently no gold colored metals or alloys that, in coins, satisfy the combination of requirements: (1) low cost, (2) appropriate density, (3) tarnish resistance, and (4) a unique, reproducible electromagnetic signature in coin acceptors for security against counterfeiting.
Cladding a gold colored metal to a low cost core solves the cost problem. Clad coinage also offers the potential of greater security against counterfeiting through a unique electromagnetic response, especially when the conductivity of the core and the cladding layers are significantly different. However, manufacturing a clad metal composite with the desired properties that are consistent and reproducible has proven to be difficult with many desirable clad and core combinations.
One coinage clad is disclosed in U.S. Pat. No. 4,330,599 to Winter et al that is incorporated by reference in its entirety herein. The Winter et al patent discloses a three layer clad with a hard copper alloy core having a yield strength above 51,000 psi. One disclosed core material is work hardened copper alloy C110 (nominal composition, by weight, 99.90% copper minimum and 0.05% oxygen maximum). Clad to this core is a gold colored alloy containing, by weight, 2%-3.5% aluminum, 1%-2.5% silicon and the balance copper. The cladding layers have a yield strength of less than 15,000 psi and are bonded to both sides of the core by cold rolling with a one pass reduction of 50%-75%.
Throughout this application, all alloy compositions are recited in weight percent unless otherwise noted.
Another gold colored coin clad is disclosed in U.S. Pat. No. 4,429,022 by Breedis et al, that is also incorporated by reference in its entirety herein. The Breedis et al patent discloses a core formed from copper alloy C151 (nominal composition by weight 0.05%-0.5% zirconium and the balance copper). The cladding layers are formed from a gold colored alloy containing: copper, 2%-3.5% by weight aluminum and 1%-2.5% by weight silicon. Bonding is achieved by a single pass reduction of 50%-75% followed by heating to a temperature of from 200.degree. C.-750.degree. C. for 5 minutes-24 hours.
The above clads provide a low cost gold color coin, but when the bonding reduction during cladding approaches 70%, the prior art clads are prone to corrugation. Corrugation is periodic ripples formed in the clad transverse to the rolling direction. Corrugation becomes worse as the composite becomes thicker and the problems with current cladding combinations are exacerbated as planchet thickness is increased from the present 0.65 mm to an anticipated 3 mm.
Corrugation is highly detrimental to automated identification of coins such as in a vending machine. As disclosed in U.S. Pat. No. 4,361,218 to Van Dort that is incorporated by reference in its entirety herein, one device to discriminate coins of different denominations emits an oscillating magnetic field. When a coin passes through this oscillating magnetic field, there is an electric signal phase shift in detecting coils that is dependent on the coin composition and coin diameter. This phase shift is electronically compared to the phase shift caused by a control coin.
Variations in the thickness of the clad layers will change the phase shift giving a different electromagnetic signature and require the acceptance parameters of the coin testing apparatus to be broadened to accept the variation between same denomination coins. Broadening the acceptance window compromises the discriminating capacity of the coin accepting device, making counterfeiting of the coins easier.
To minimize mistaking a coin from another one of a different denomination or nationality and to make counterfeiting more difficult, a gold colored coinage clad that is not prone to corrugation is needed to narrow the acceptance window of automatic coin discriminators.